Friday, June 24, 2016

Making Dromaeosaurids Nasty Again Part II - No Shame In the Scavenging Game

Scavenging gets no respect. There is a reason you don't see hyenas and vultures put on coats of arms or have sports teams named after them. Lions and eagles yes, but never vultures and hyenas - ironic that all of these animals do scavenge (and hunt to various degrees). Doubly ironic is that in carcass disputes it is often vultures and hyenas driving the eagles and lions off of carcasses. It is a pet theory of mine that what makes an animal an exquisite and refined hunter often works against them in carcass disputes. Because the refined hunter depends heavily on preserving the physical armaments that allow it to do its job it is at a disadvantage compared to more generalized opportunistic competitors who can be more reckless in battle. To make the sports analogy it is the difference between the tactical and technical boxer whom excels in the specific and controlled environment of the boxing ring and the unrefined street brawler. The highly skilled and nuanced technical abilities of the boxer will win the day over the street brawler in the ring for sure. But take this tactician out of the ring and into the street against the brawling street fighter - who fights dirty, uses bluff and swagger, and can simply take chances with regards to life and limb that the professional boxer can't - and the outcome swings in favor of the brawler. What makes a good street fighter work well is a different skill set than what a boxer has and what makes a good scavenger work well is a different skill set than what a specialized hunter has.

Cue youtube videos:

Note how the tawny eagle just can't hang with the vulture hordes. A better hunter the eagle definitely is - but a better brawler than the vultures? Sorry the eagle just can't bang with the vultures.

Of course I have to include a clip of my hometown hero the California condor displacing the largest eagle of North America the golden eagle. "Step up to me Mr. Eagle?!? Pwwwfff my ancestors used to go to toe to toe with teratornids over mammoth entrails! Go hunt some rabbits!!"

Don't forget to include mention of the epic dominance of cinerous vultures (sometimes called Eurasian black vulture not to be confused with the New World black vulture) Aegypius monachus which often usurps the golden eagle.

Were dromaeosaurids more the vulture or eagle in carcass disputes? Given that dromaeosaurids did not rely on flight to hunt and may have been afforded wing pummeling; that they were literally armed from head to toe in terms of combative weaponry; that they were good sniffers; that we have evidence very suggestive of scavenging; and the ecological imperative to do so; dromaeosaurids were likely very good facultative scavengers. Yeah, so I lean heavily towards more of a vulture than an eagle in these animals. Both in appearance and demeanor.

Taking the theme that I started with on my last post - an over reliance of the import of the "killing claw" in all endeavors dromaeosaurid and refining the dominant RPR model of a "ground hawk" dromaeosaurid - I am going to expand on that in terms of how and why dromaeosaurids were very good, capable, and successful facultative scavengers. The caveat being, I should just explicitly state, that this does not imply a lack to or unwillingness to hunt. But one more thing about that hypertrophied second digit - the "killing claw". It seems several vultures sports such an enlarged second digit - the magnificent cinerous vulture (Aegipius monachus) and the red-headed vulture (Sarcogyps calvus) pictured below.

red-headed vulture (Sarcogyps calvus) credit Dibyendu Ash CC3.0

cinerous vulture (Aegypius monachus) CC3.0 credit Mistvan

Enlarging the toe on the second digit serves a useful purpose in pinning meat down which allows the beak and head to get leverage in pulling bites off. The "killing claw' in maniraptorans might have been used just as much if not more in leveraging good bites while scavenging/feeding as opposed to outright "killing". Additionally the line of action on the dental serrations suggests that the denticles on the back end of the tooth - the lingual side - were doing most of the cutting work as the animal pulled back on food items. It really is all about the teeth and jaws anyways...

First things first. The issue of ground based scavenging. We have all heard the argument "you can't have an obligate ground based scavenger because there simply isn't enough carcasses to sustain such an animal". Sounds reasonable enough and this was one of the trump arguments used to disavow the scavenging T. rex hypothesis. Is there a way to test it?

Some researchers recently did just that in a particularly ingenious and creative manner: Body size as a driver of scavenging in theropod dinosaurs (Kane et. al., 2016). Under various conditions of carcass detectability, carcass size, and competition they performed basically a SIMS version of let different sized theropods find dead stuff and see what body size pans out to be the most efficient in terms of cost/benefit analysis. Basically if you are looking for carcasses to supplement your diet the results are congruent with the assertion that only soaring scavengers can make a total living off of scavenging. Both small theropods and large ones had difficulty earning enough calories from facultative scavenging to make the activity worthwhile. If we think about the issues faced by both very small and very large theropods in terms of garnering benefit from scavenging this makes sense. Small scavengers can't cover as much ground and are easily displaced from carcasses. Large theropods - although they can dominate a carcass - have to move all that weight around and by the time that they detect and reach a carcass much of the available calories might already be consumed by smaller and more numerous scavengers.

What the researchers came up with is very interesting in terms of optimal body size for a land based scavenging theropod. There appears to be a mid-sized sweet spot. Small fry like microraptorines didn't do so well and at the other extreme multi ton giants like tyrannosaurids and carcharodontosaurids did not fair too well as facultative scavengers. Facultative scavenging is most optimal in terrestrial theropods in a size range of between (they get pretty specific) 27 and 1,044 kilograms.

Dromaeosaurids slot into that size range very nicely you should note.

While I have read of several people poo-pooing this paper and its conclusions I personally find a lot of merit (with some caveats of course) in this methodology and the conclusions reached. At the lower end of the size scale I would think that Velociraptor and Dromaeosaurus would still fair pretty well even though they are lower than 27 kilograms. Jackals and coyotes do pretty well as factulative scavengers and they are a bit smaller than 27 kilograms. Additionally at the higher size range let us keep some perspective. Imagine just >one< 50 ton sauropod died in an ecosystem with several multi-ton theropods and loads of smaller and immature theropods. Chances are the big theropods could locate, dominate, and feast on that sauropod for a while. Their large size and relatively slower metabolism might allow them to scarf down a load of meat that sated them for several weeks or even months. So the big boys might scavenge less frequently but when they do dominate large carcasses that glut of food might form the bulk of their caloric intake for quite a while.

Not to mention this paper completely jives with the conclusions of a similar paper that came out in 2011 that has gone a little overlooked... when two independent studies reach similar conclusions that should perk your interest. The 2011 paper is called Intra-guild competition and its implications for one of the biggest terrestrial predators, Tyrannosaurus rex (Carbone et. al. 2011).

Here is my favorite excerpt from the results:

Physical evidence suggestive of scavenging

Dave Hone has written a bit on Velociraptor consuming and - in his interpretation - likely scavenging azhdarchid pterosaurs here and here. Make sure to go back and read the commentary it is hardly a close and shut case of scavenging. Hone makes a more compelling case for Velociraptor scavenging a hefty size Protoceratops "fighting dinosaurs", part II. Hone also has some papers on the topics but they are not open access, shucks. The blog posts and commentary get the point across though.

credit Brett Booth

Phil Currie and colleague (1995) also describe another azhdarchid consumed by a velociraptorine theropod but don't conclude it was scavenged.

I don't fully agree that the idea of a velociraptor taking down a 2-3 meter wingspan pterosaurs is nonsensical so for me that is equivocal proof of scavenging. However the scraping of flesh off the jaw of the large Protoceratops is a more compelling argument for scavenging.

Additionally you have the famous Yale Deinonychus and Tenontosaurus quarry that might indicate not only scavenging but aggressive intraspecific killing and cannibalism among Deinonychus. And who knows what surprises await us with the new Utahraptor block James Kirkland and co. are working upon...

Roach and Brinkman (2007) advocate a "diapsid like" foraging strategy in Deinonychus and other theropods as the best and most parsimonious null hypothesis. They eschew the notion of cooperative pack hunting posited to bring down herbivores that are several orders of magnitude larger than the theropods but instead highlight scavenging, combat, and cannibalism as the likely culprits that resulted in the taphonomic signal from the Yale quarry. Chief among these is the presence of articulated tails in the quarry the argument being that the bony and tendonous tails were eschewed in favor of more meaty pieces that were hauled off. I do agree with a lot of what they are selling. They do leave the opening for opportunistic group foraging where a congruence of factors might lead to a "mobbing" type foraging scenario. We see this with sharks gathering escorting ailing whales along and literally eating them alive. Or crocodiles gathering at choice feeding situations or latching onto the same prey animal or loosely "cooperating" to shoal prey together. I would also extend this to the manner in which vultures gather around a carcass and use strength in numbers to harass and intimidate other predators off a carcass.

My purpose here is not too weigh too heavily on whether each of the above scenarios are unequivocally evidence of scavenging but merely highlight that there is a body of evidence that points heavily in the direction of scavenging in dromaeosaurids.

On Hell (Creek) Patrol With Dakotaraptor

Few stories in dinosaur paleontology were as exciting and celebrated as the revelation of an honest to goodness mega-dromaeosaur in Dakotaraptor (DePalma et. al., 2015) that inhabited the latest Cretaceous of North America right alongside good ol' sexy rexy. Met with a flurry of fanfare and fan art I decided to let the dust settle a bit before giving this beast the antediluvian salad treatment.

Any epic beast needs a suitably epic soundtrack to go along with it - Judas Priest's Hell Patrol:

Like wild fire comes roaring

mad whirlwind burning the road

Black thunder white lightning

Speed demons cry the hell patrol

There are a few points from the paper on Dakotaraptor that have not received enough attention in my opinion. Specificities that really speak to the adaptations and ecology of an animal that should garner much attention.

Dakotaraptor Is Cursorial
Interpretations of Dakotaraptor that highlight the ecology of this animal as a cryptic, stalker of closed habitats - the "ghost of the forest" scenario - are missing the story that the bones are telling us. This animal was leggy, it was cursorial, and in life it would have departed strongly from the more low slung and slower Utahraptor or Deinonychus.

credit taphonomy CC4.0

I mean just check out those legs!! Keep in mind that this critter was having to go toe to toe with ridiculously leggy young T. rex and you can imagine the evolutionary imperative to get speedy. Dakotaraptor appears to have made the necessary concessions needed to increase cursorial ability.

As I discussed in my last post there are certain compromises to be made in terms of raptorial grasping abilities in these animals versus cursorial ability as highlighted by Fowler. To review as grasping ability increases the metatarsals shorten for better leverage and strength but running ability is compromised; conversely as cursorial ability increases grasping strength decreases. Also bear in mind that all dromaeosaurids/maniraptorans were relatively weaker than modern raptors in terms of grasping power of the feet - they were not simply overblown red-tailed hawks.

An Interesting Incongruity
If you followed my articles on Spinosaurus and my argument for underwater punting in that animal you know that I do have a fondness and attraction for anatomical incongruities; two things combined that seem to make no sense but when analyzed in a different light actually offer a more refined animal. In Dakotaraptor we have such an incongruity.

Digit II is hypertrophied. The "killing claw" in this animal lives up to all the hype. Not only is the claw relatively large, the attachment for the muscle and tendons that drive it - the flexor tubercle - is robust. Whatever way in which this animal was using its digit II it is obvious that it has invested heavily in it.

However the incongruity is seen when we move out to digit III and IV where the claws on those digits are not highly recurved nor is there a significant flexor tubercle present. In fact the flexor tubercle is so reduced in those digits to be pretty much non-existent!!

I mean just look at the contrast in the flexor tubercle attachment in these two claws. For me this is a compelling argument that the classic raptor prey restraint model posited for these animals is lacking. Dakotaraptor was moving further away from grasping things with its foot claws yet at the same time digit II was still large and strong. What is going on here?

Going back to my earlier bit on the cursorial aspects of this animal and resolving its place in the ecology of the environment in which it lived proves useful I contend. Several inferences are useful in giving better precision as to how this animal likely operated.

Inference 1Dakotaraptor was highly cursorial.

Inference 2Dakotaraptor - at least in the adult morph - was fairly limited in arboreal capacity. At best they were probably very clumsy in trees due to large size, long legs, and limited foot strength.

Inference 3 Lesser dromaeosaurids, azhdarchids, and tyrant lizards - especially juvenile and rapidly growing teenage rexes - created an especially quarellsome predatory and scavenger ecology. All of these animals would have been usurping, competing, and fighting over carcasses. Azhdarchids, juveniles rexes, dromaeosaurids, and Dakotaraptor all slot into the ideal size niche to be successful facultative scavengers.

Inference 4Dakotaraptor could dominate pterosaurs, dromaeosaurids, and smaller rexes. However because it could not dominate larger immature rexes - which were still very cursorial - Dakotaraptor itself needed both speed and maneuverability to evade these animals as it was likely not highly arboreal.

Inference 5 The enlarged and robust ungual and claw in digit II primarily aided in combat - especially in competitive skirmishes over carcasses - and assisted in pinning meat down as the teeth, jaws, and neck pulled bites away from the animal (alive or dead). The denticles on the teeth of Dakotaraptor are most defined on the rear (lingual) side as they are in other dromaeosaurids supporting this notion, The RPR restraint model appears less important in this animal relative to other dromaeosaurids in prey capture. Instead digit II could primarily be used in combative endeavors while wing pummeling and the teeth and jaws did the actual killing.

HellPatrolDakotaraptor by Duane Nash

Dakotaraptor may just be a very specialized and rare component of the Hell Creek hunter/scavenger guild. It was large enough and well armed enough to drive off everything but the larger rexes from a carcass. The cursorial adaptations speak to an animal that could cover a lot of ground to find carcasses, prey, and evade larger competitors if need be. In short we see a lot of compromises that facilitated the existence of a sometimes hunter often times scavenger. It was big, but not so big that scavenging became impractical. It was well armed with a massive and powerful digit II but compromised the raptorial abilities of the other digits in favor of cursorial adaptations. The forelimbs were large, feathered, and powerful - great assets for combat, display, and intimidation. In many ways the large and rare Dakotaraptor was possibly like the lappet faced vulture of Africa. Large and strong enough to dominate most other scavengers at a carcass but would step aside for the larger tyrant lizards just as lappet faced vultures dominate other vultures, marabou storks, and jackals but move aside when hyenas show up.

Wings Suggested as Dominance Display

As in my last post on wing pummeling I will get behind another spotential behavior for dromaeosaurid wings that gets surprisingly scant attention - dominance display.

Such wings could be spread wide over carcasses in order to appear large and more dominant over competitors. Indeed this is the classic show of dominance modern carcass rendering birds (vultures, raptors, petrels) display at carcass disputes. Wing display for intimidation is so painfully obvious I can't believe that such suggestions don't get more attention. I have heard of it before so I can't claim to be the first to suggest it but I am definitely in favor of it. When we look at what modern carcass disputing theropods do with their wings... you have heard that story before though.

Such a shame that vultures do not get the recognition and respect they deserve for their role in modern competitive ecosystems; that they are spiralling into extinction; that they are not posited as the go to analogy for how extinct theropods looked, behaved, and operated when rendering carcasses - despite the fact that, quite simply, they are the modern theropod that does such things most often. Instead grumble, grumble flipping venomous lizards for crying out loud. Venomous lizards.

When you are watching a scrum of vultures feeding you are witnessing a direct portal into to the feeding activities of dromaeosaurids and other theropods in the Mesozoic's past. The theatrical displays; the spread wings; the fast jerky bites; the constant fight for dominance and hierarchy; and, most importantly, the constant and non-stop flurry of motion, action, and violence. Such scenes would be enacted on a much larger and grander scale in the Mesozoic. A difference of degree but not type. A sight both startling and jaw dropping.

Coming up: restoring dromaeosaurids, ugly vs. pretty, and a new hypothesis on dromaeosaurid biting technique!! P.S. I will talk of the weird "tail plume" I depicted on the Dakotaraptor in my next post too so don't even ask about it.

19 comments:

Congratulations on making Dromaeosaurids my favourite group of (non avian) Theropods again! These last two posts have changed my general opinion of Dromaeosaurs a lot. I once thought of them as graceful assassins like modern hawks, but now I look at many species as giant predatory vultures, complete with massive, disgusting wattles and dare I say even quills on certain parts of the body. You truely are making Dromaeousaurids nasty again! So seeing as how medium sized species were usually "carcass bullies", what would smaller sized species like Velociraptor be like then? Would they still just be scavengers and/or small game hunters? And what of the massive Utahraptor? Although it fits the general size plan for an ideal terrestrial scavenger, it lacks long legs or any major cursorial adaptations. I mean, jsut look at how stumpy the legs are:http://pre04.deviantart.net/63c2/th/pre/i/2015/313/e/d/dakotaraptor_and_utahraptor_2_0_by_hyrotrioskjan-d9fzc6v.jpgI assume it was primarily an ambush hunter then? Seeing as how it couldn't really track down carcasses for miles like the others.

Amazing post as always. I never heard that Dakotaraptor had such small III and IV foot claws at least in the adult form (That is probably the reason why Saurian decided to have adult Dakotaraptors not climb trees).

"What would velociraptor be like?" Probably a mixed bag of scavenging and hunting. It was likely small due to its environment but still probably a bit of a meso-predator like a coyote. Tsaagan likely bullied and even killed velociraptor though.

I want to hold off on Utahraptor until the new stuff is revealed. I have heard there is some unexpected stuff with the head... I don't want to prohibit them from scavenging. We don't know enough about the efficiency of their walking to say for sure that they could not cover distance albeit slowly. Like bears do which are plantigrade but still cover huge distances foraging and scavenging.

@khalil beiting Velociraptor and other small dromaeosaurs probably engaged in "tail pulling" whenever they come across a larger carnivore at a carcass, in which they pull the tail feathers of the larger dromaeosaurs so that they can release the food. The method is seen in modern days corvids and is in fact a very common behavior.http://corvidblog.tumblr.com/post/37622242234/tail-pullinghttps://www.youtube.com/watch?v=JPB2TikKfPA

Oh yeah, i love this article!I mean, dromies as vulture analogues (and it's been years since i started to look at 'em this way) which, by the way, are some of my favorite modern dinosaurs; my fav among them are the griffon (Gyps fulvus) and the mighty lammergeier (Gypaetus barbatus)... Aah, the beauty of these raptors, i could talk about 'em for hours!

On the other hand, i seriously hope Kirkland & co will soon unveil what they discovered about the Utah dromie, i can't wait any more! I heard stories about short tails, big heads, curved lower jaws... Man, how long we have to wait for the friggin' paper?

@Alessio Right... how long do we have to wait? Probably a while. Should start a bet on which comes out first the Spinosaurus monograph or Utahraptor paper. What irks me is that I suspect tons of people have seen the block of Utahraptor and shared images privately. But us poor plebeians have to wait. I kind of get the secrecy but kind of think it is lame... what are they worried about someone else getting the scoop on their specimens and publishing off a photograph on the internet? I am sure they are taking their time as not to disturb or lose data so get prepared for a long wait...

@Duane Nash Yeah, it will probably still be quite a while before the Utahraptor paper is finally published, so until then the best we've got is Scott Hartman's silhouette (which, luckily, reveals quite a bit). On the bright side, "Lori" is finally going to get published this year and the paper Scott did an Argentinosaurus skeletal for is supposed to get published this year as well.

By the way, I forgot to ask, but this has always been a great question I've been wondering of, but is there evidence in favour of flight being ancestral to Paraves? I always just assumed it was since basal Paraves, basal Dromaeosauridae and avians were all fligh capable, and derived Dromaeosaurids share a lot of traits in that juveniles are flight capable. But Troodontids don't really show anything flight related, and the Dromaeosaurid Mahakala doesn't either, yet it's incredibly basal. What is the most likely thing going on with the origin of flight? And what arguements are on both sides (side A being flight is basal to Paraves and side B being it was evolved multiple times)?

@khalil I dunno. Things will be in a flux for a while. Seemed for a while like a new dino-bird critter was coming out of China like every week. I don't think we have properly digested and put together the things we do have much less what we don't have.

Agree with ya, Duane, while i can understand why they wait so long to release the paper, on the other hand, well, i don't see anything inherently bad if the photos of the specimens are available for the public some time before the official publication... After all science should teach and share.Oh well, i'll stop here before someone start to think mine's a rant against scientists or something like that, which is definitely NOT the case ;)

@khalil beiting Actually, it seems that the latter is the most likely, since many paravians once assumed to be flight capable are now believed not to be (such as Archaeopteryx and Anchiornis). Large wings however do seem to be ancestral to paravians, whether they could fly or not, since Archaeopteryx, Zhenyuanlong, Velociraptor, and Dakotaraptor all had very large wings but were incapable of flight.

A small caveat to the first paragraph, from what I found in two hours of research lions and hyenas seem to have about equal rates of stealing from each other (when averaged across all populations), hyenas also seem to do more hunting proportionally to what lions do over all, so the lions are the bigger scavengers really. That said hyenas as a general rule will not attack a kill that a male lion plus pride is at simply because the male is so big and strong(images of tigers single handedly fighting packs of wolves comes to mind).(My sources are the African journal of ecology, Botswana.co.za, and some sources from Wikipedia )

Also about the whole eagles v vultures caveat. At first i would have thought that eagles would win at kills, obviously that's mostly wrong with your first vulture terror bird post. But when I further researched it, well I really can't see why the large vultures both new and old world would ever lose. Most old world vultures are equal to larger then all current living Eagles by five to ten pounds plus and from my general observation kill dominance is in part based on who is the bigger bully(amongst other things) and not something about specialized anatomys (usually). Given that Eagles do dominate all of the new world vultures with the obvious exception of the large condors.

It seems to me that the only living contenders here for a relatively fair contest are the largest living Eagles and the smaller end of old world vultures with the ruppell vulture standing out as a ideal choice, my particular choice for the Eagles would be the stellar sea eagle as it is in some respects quite the scavenger and known for being quite the aggressive kleptoparasite, this followed by it being equal to slightly larger then ruppell. This I think would help establish a more definitive vulture vs eagle. Sadly there is no way we could humanely test this and they don't live any where's close to each other and lack history of each other.... Anyways there ends my long caveat.

P.s not saying anything about scavengers being lesser then other lifestyles and such just that your first paragraph swerves off the mark from the rest of a damn fine article on speculative Mesozoic ecology etc.

If anyone wants to read a relatively recent paper on vulture interspecific conflict( especially against raptors and other vultures) It's quite the little gem.

"Vultures: their evolution, ecology, and conservation" 2015

It covers everything in the title for both new and old world vultures, truly fascinating read, it's on Google books if your wondering where to read it, it cost 103 $ for ebooks but you can read the stuff on vultures and raptor interspecifics between page 136 to 148 and more for free, there's probably more you can read on the inter species relationships but I don't know if you can read the whole book for free or not.

Hi I was thinking of posting this comment in your part three article but thought this was more topic apporpiate place to do so, if you are alright with me posting something like this in a less related but more recent article that would be a nice clarification.

But back to the point, given that Dakota raptor was a curosial lappet faced vulture with greater predator inclinations well, the next thing that strikes me as being similar to this style of creature would be the various abelisaurs of South America and maybe North Africa. The idea is that given there was a lot more giant titanosaurs in SA there would be larger Dakota raptor type Dino's and given that abelisaurs fit this niche quite well being so curosial and large etc. Sure the largest abelisaurs would probably not fit here but given your original consternation as to the whole what hell were abelisaurs ( this is relation to your falcon or vulture allosaurus article) doing for ecology well perhaps they were the 2nd (or maybe first) responders to the many large carcasses. This along with the many smaller abelisauroids acting like raptors in niche seems awfully uncanny.

Vultures may be fierce, but when I think of raptor scavenging behaviour, I think of the caracara. They have been known to bully even vultures at carcasses, and they hunt and nest on the ground like dromaeosaurids.